This invention relates to a carrier for use in electrophotographic two-component developers comprising the carrier as a component together with a toner component. In particular, the present invention relates to a coated carrier having excellent durability in the magnetic brush developing process of electrophotography.
In one of the electrophotographic methods, a dry, two-component developer comprising a toner component in combination with a carrier component is used in the magnetic brush developing process to develop a static image formed latently on a light sensitive substrate. Commonly, the developer comprises toner particles having sizes in a relatively fine range and carrier particles having sizes in a relatively coarse range. The static attraction between the opposite polarites generated by contact of these particles holds the fine toner particles on the surface of the coarse carrier particles. When the thus statically charged developer is brought into contact with a latent static image which has been formed on a light sensitive substrate, part of the charged toner particles are statically attracted and transferred to the latent image to produce a corresponding visible image. Therefore, the toner particles should have an appropriate triboelectric property so that they hold a sufficient charge to ensure the precisely selective transfer thereof to the latently imaged area.
When the conventional dry, two-component developers are used for a period of time in the electrophotographic developing process, there has been a tendency that the toner or dust thereof soils the surface of the carrier particles to eventually form a stiff continuous film of toner material on said surface due to repeated contacts and impingements between the carrier and toner particles and between these particles and mechanical parts of the developing apparatus. Once such a film has been formed, the accumulation of toner material on the respective carrier particles becomes gradually heavier. Then, the triboelectric charge of developer which has been generated by contact of the bare surface of the carrier particles with the toner particles is replaced by the triboelectric charge which is now generated by contact of the toner-coated surface of the carrier particles with the toner particles, i.e. by toner-toner contact. Thus, the triboelectric property obtained by the fresh developer is seriously impaired. Consequently, the quality of the final reproduced copies becomes poor due to the soiling of the back ground with a significant amount of toner resulting from the impaired triboelectric property.
Hitherto, it has been proposed to remove or partly obviate the above difficulties by modifying the carrier, for example, by coating the surface of the carrier particles (or the core material) with a material having low surface energy, such as silicone resin (see for example Japanese Patent Publication 44-27879 and Japanese Patent Public Disclosure (KOKAI) 50-2543). With the proposed techniques, the undesirable soiling and accumulation of the toner material on the surface of the carrier particles are inhibited considerably. However, in the product carrier coated with a silicone resin, generally the coating layer has poor adhesion to the core materials, such as iron powder. Also, the coating layer exhibits mechanical strength insufficient to withstand well the friction and shock to which the developer is subjected during the use. Therefore, in a continuous operation for a long period of time, the silicone layer coated on the surface of cores becomes worn out or detached from the cores. Thus, the cores with the bare surfaces are exposed to the toner. Then, the initial triboelectric charge which has been generated by contact of the silicone-coated core particles with the toner particles is replaced gradually by the triboelectric charge generated by contact of the bare core particles with the toner particles. Accordingly, the initial triboelectric property of the fresh developer again can not be maintained at a constant level during prolonged use and the resulting variation in the triboelectric property will adversely affect the quality of reproduced copies.